Calculate and retain "blockified" version of trigger in dedicated property

[tsalo]

Detailed Description

This issue stems from @smoia's comment in https://github.com/physiopy/phys2bids/pull/219#discussion_r457513658. Essentially, the BIDS-synchronized run labeling and splitting workflow in #219 requires onsets of physio runs before the workflow is called (durations are useful for visualization, but not necessary). General splitting would also benefit from having a binarized version of the trigger, with any signal variability averaged over the course of each run, in order to identify the onset and duration of each run.

Context / Motivation

This is useful, and perhaps necessary, for the workflow in #219, but also is helpful for splitting runs regardless of general workflow, as the run onsets and offsets are the basic information needed for the task.

Possible Implementation

I don't know. For natively block-like triggers like mine, this is a non-issue, but I'm hoping that the rest of the devs have a good way of determining run onsets and offsets for variable triggers that can be applied here.

[smoia]

@physiopy/phys2bids does anyone wants to help with this issue in order to unblock #219?

Ideas are welcome!

I think we can apply something similar to BlueprintInput.check_trigger_amount - maybe even that same function, expanded a little.